The invention relates to a supplementary steering system, especially a rear wheel supplementary steering system for passenger motor vehicles having a regulating device which deflects the steered wheels, or the steerable rear wheels as a function of a quantity correlated with vehicle longitudinal velocity, transverse acceleration and yaw velocity, in the sense to stabilize the vehicle.
A similar supplementary steering system is known from German Unexamined Patent Application DE-OS No. 33 00 640. That supplementary steering system is intended to compensate the disturbance movements of the vehicle during the reactionless time of the driver.
It is indeed pointed out, quite generally, in German Unexamined Patent Application DE-OS No. 33 00 640 that the control of the supplementary steering system can take place as a function of the longitudinal velocity, the transverse acceleration and/or the yaw velocity. However, this publication does not contain any precise instruction as to how unstable vehicle conditions, which can lead to skidding movements, are to be detected in due time and stabilized in an optimal manner by the supplementary steering system.
Thus, the object of the invention is to provide a supplementary steering system which is especially suitable to achieve a particularly effective stabilization of the vehicle in the event of an increased risk of skidding.
This object is achieved when the occurrence of a skidding movement of the vehicle associated with a rapidly increasing transverse velocity of the vehicle occurs. A regulating device deflects the steered rear wheels in a sense of counter-steering for counteracting the skidding movement. Also, the respective lock angle of these wheels is controlled as a function of the change of vehicle transverse velocity, or of a quantity correlated therewith.
The change of transverse velocity is determined by: v.sub.y =a.sub.y -v.sub.x .psi., where a.sub.y represents the transverse acceleration, v.sub.x represents the longitudinal velocity, and .psi. represents the yaw velocity of the vehicle.
The invention is based on the finding that a rapid growth of the transverse velocity of the vehicle is characteristic of the commencement of skidding movements of a vehicle. Accordingly, the fundamental concept of the invention consists in counter-steering with the steered wheels, especially the rear wheels, in the event of the occurrence of changes of transverse velocity and thus preventing, or at least slowing down, the incipient process of rotary movement of the vehicle. The supplementary steering system invariably becomes effective when changes of transverse velocity occur. As soon as the transverse velocity of the vehicle is no longer changing, the steering lock, caused by the supplementary steering system is withdrawn again, so that the steering range, which is present for the stabilization of critical vehicle conditions of the supplementary steering system, is again fully available for new critical situations. Advantageously, at the same time, the driving behavior under steady driving conditions remains unchanged as compared with a vehicle without a supplementary steering system.
Additionally, or alternatively, it can be provided that the deflection of the steered or rear wheels is made as a function of the change of transverse velocity as well as of the change with time (rate of change of the change of transverse velocity).
Furthermore, according, to a preferred embodiment of the invention, a theoretical value for the change of transverse velocity of the vehicle can be determined from travel speed and manually set steering deflection can be compared with the respective actual value, and with the regulating device operating as a function of the difference between theoretical value and actual value; and in this embodiment, it is guaranteed that the driving behavior is unchanged as compared with a vehicle without a supplementary steering system in non-critical situations. Accordingly, a driver would not require any acclimatization period on transferring to a vehicle with a supplementary steering system, since the latter becomes effective only in critical situations in the sense of an automatic stabilization of the vehicle and thus in the sense of a facilitation of the control of the vehicle.
It is advantageous to have the deflection of the steered or rear wheels be as a function of the change of vehicle transverse velocity as well as of the change with respect to time (rate of change of the change of transverse velocity). Also, theoretical values of the change cf transverse velocity can be determined from travel velocity and manually set steering deflection or the like, and this theoretical value is then compared with the actual values by a computer and the regulating device is then made to operate as a function of the difference between theoretical and actual values. Supplementary sensors can be arranged for the direct or indirect measurement of the friction value between tires and roadway and are used to influence the control function in response to the vehicle transverse velocity.
It is also advantageous if the steering movements of the supplementary steering system take place independently of the instantaneous steering angle of the front wheels and of the steering activity of a driver, respectively.
Ideally, the steering deflection caused by the supplementary steering system, for the steering angle of the rear wheels (.delta..sub.HA) should be determined by a computer as follows: ##EQU1## and T.sub.H is a predetermined time interval;
K.sub.p, K.sub.D and K.sub.s are predetermined constants; PA0 t is the time; PA0 v.sub.y is the change of transverse velocity; PA0 v.sub.y, actual is the actual value of the change of transverse velocity; PA0 v.sub.y, theoretical is the theoretical value of the change of transverse velocity, and PA0 .tau. is a time variable. PA0 l.sub.H is the distance between the rear axle of the vehicle and the center of gravity of the vehicle (S.sub.p); PA0 l is the wheel base of the vehicle; and PA0 .delta..sub.VA, .delta..sub.VA are the steering lock and the change of steering lock of the front wheels respectively.
Under such a system, the theoretical value of the change of transverse velocity (V.sub.y, theoretical) is determined by a computer as follows: ##EQU2## where v.sub.x, v.sub.x are the longitudinal velocity and the change in longitudinal velocity respectively;
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.